1. Field of the Invention
The present invention relates to temperature detection processing of a fixing member.
2. Description of the Related Art
In electro-photographic image forming apparatuses, a toner image transferred onto a recording material is fixed by a fixing device. The fixing device includes a fixing member having a heater, and a sensor for detecting a surface temperature of the fixing member. To keep the surface of the fixing member at a temperature (target temperature) at which the toner melts, based on the surface temperature of the fixing member detected by the sensor, a power supply of the heater is controlled.
As the sensor for detecting the temperature of the fixing member, a noncontact temperature detection element is used so as not to damage the surface of the fixing member. Japanese Patent Application Laid-Open No. 2003-57116 discusses a noncontact temperature detection device having a film for absorbing infrared rays emitted according to a surface temperature of a fixing member, and generating heat according to the amount of the absorbed infrared rays. The temperature detection device detects a surface temperature of the fixing member based on a value obtained by subtracting a temperature of an infrared absorbing film supporting member detected by one thermistor, from a temperature of the infrared absorbing film detected by another thermistor. More specifically, a surface temperature of the fixing member is determined based on a temperature difference between a detected temperature of the infrared absorbing film and a detected temperature of the supporting member, and the detected temperature of the infrared absorbing film, by referring to a data table. The temperature difference between the detected temperature of the infrared absorbing film and the detected temperature of the supporting member is calculated because an analog-to-direct (A/D) converter converts an output voltage of the thermistor element from an analog value to a digital value, and thereby the ability (resolution) to detect surface temperatures is restricted.
In this technique, the analog circuit is designed on the assumption that the temperature of the infrared absorbing film is higher than the temperature of the supporting member, and further, the data table is determined based on this assumption. Consequently, when the detected temperature of the thermistor provided on the infrared absorbing film is lower than the detected temperature of the thermistor provided on the supporting member, the device makes an incorrect decision that one of the thermistors is out of order. Consequently, when the temperature detected by the thermistor provided on the infrared absorbing film is lower than the temperature detected by the thermistor provided on the supporting member, the device may not detect the temperature of the fixing member.
For example, when the fixing member is replaced, the temperature of the infrared absorbing film rapidly decreases. On the other hand, the temperature of the holding member gradually decreases. That is, it is possible that, immediately after the replacement of the fixing member, the temperature detected by the thermistor provided on the infrared absorbing film may be lower than the temperature detected by the thermistor provided on the supporting member.